1. Fields of the Invention
The present invention relates generally to a plunger that is used in a resin molding device for encapsulating electronic components, and, more particularly, to an improved plunger that is designed and constructed so that the molding apparatus encapsulates electronic components more economically, efficiently, and effectively.
2. Discussion of Background and Prior Art
Resin molding apparatuses for encapsulating electronic components, such as, semiconductor devices are well known. A standard resin molding apparatus typically includes a mold assembly that has an upper mold element interfacing with a lower mold element which provides a mold cavity in the interface between the two elements. A set of electronic components, such as a semiconductor device supported at its outer periphery by a lead frame, is placed inside the mold cavity. The lead frame is in turn supported on an edge in the mold cavity. A cylinder, for moving resin into the cavity is located in the molding apparatus near each mold cavity.
The cylinder is pre-heated to a certain temperature and then a thermosetting resin tablet is inserted into the lower end of the cylinder beneath the plunger and is further heated until it is plasticized. The plasticized resin flows to a pot. Typically, a passageway (i.e. "cull") or runner in the interface between the mold elements links the cylinder with the mold cavity. Sometimes the cylinder and plunger are in the upper mold element which closes directly onto a mold cavity located directly beneath the cylinder in the lower mold cavity. The plasticized resin flows in the cull from the cylinder to the mold cavity to encapsulate the semiconductor devices.
The motive force for getting the plasticized resin to the mold cavity is a plunger slidably mounted within the cylinder which contacts and pushes the plasticized resin through the passageway to each mold cavity. A reciprocating means is typically mounted to a top side of the plunger for reciprocating the plunger within the cylinder. After the resin reaches the mold cavity encapsulating the semiconductor devices, the mold is opened, the devices removed, and the resin cured.
One problem with the prior art molding apparatus is that the plungers are typically designed with flat bottom surfaces. When a flat surfaced plunger is pushed down in the cylinder against the plasticized resin, it causes a turbulence in the flow of the resin underneath the cylinder and within the cull. The turbulent resin is not smoothly directed toward the cavity and much of the resin tends to flow to undesired areas in the molding apparatus instead of to the desired mold cavities. Specifically, the turbulent or outward flow of the resin at the bottom side of the plunger causes some of the resin to flow into interfaces between the upper and lower mold elements and into the interface between the outer surface of the plunger and the inner surface of the cylinder between which there is a sliding clearance fit.
Resin that flows into these interfaces causes difficulties in opening the molds because the resin cures quickly, and this curing of the resin in these interfaces results in excessive cleaning costs and frictional resistance to the sliding plunger during its operation. Since the plunger does not slide as freely or efficiently, the frictional resistance also results in excessive wear of the plunger requiring more frequent replacement and excessive down-time for the mold.
Resin that flows into the undesired interfaces results in still further problems. More resin in the undesired interfaces means less resin flowing into the mold cavity. A defect in molding, therefore, may occur because insufficient amounts of resin may not reach the mold cavity. Additionally, the resin in the interface can begin to harden and cure if the mold is not opened quickly, causing further resistance to retracting the sliding plunger in the cylinder with the reciprocating means. Often the reciprocating means is physically pulled out of its attachment to the plunger resulting in damage to the plunger requiring shutdown until repairs can be made.
Some prior art plungers provided a groove in the lengthwise surface of the plunger to create an enlarged clearance in the interface between the plunger and cylinder as a means to allow any excess gas or resin to flow into the interface, thereby providing an outlet for the excess gas or resin. The problem with using such lengthwise surface grooves was that the amount of excess gas or resin that the plunger surface could receive was limited by the sizes of the grooves. Some prior art plungers were designed to provide enlarged clearances at the plunger-cylinder interface by having a smaller diameter upper portion of the plunger and a larger diameter lower portion of the plunger. The problem with such a plunger, however, was that the plunger would wobble in the cylinder and could not be stably guided through its reciprocation in the cylinder because of the differential in diameters of the upper and lower portions. The wobbling aggravated the condition of having excessive resin flowing into the piston-cylinder interface.
Therefore, there is a need for a plunger used in a molding apparatus for encapsulating semiconductors that causes resin to optimally flow towards the desired mold cavities and away from undesired areas, such as, the upper-lower mold element interface and the cylinder-piston interface described above. Furthermore, there is a need for a plunger that provides more outlets for resin that flows into the piston-cylinder interface but is still able to stably guide the plunger through its reciprocating path in the cylinder.
In overcoming these problems and limitations of the prior art, it is an object of the present invention to encapsulate electronic components in a resin molding apparatus wherein the apparatus includes a plunger design that provides greater capacity to receive excess gas or resin that does manage to escape into the interface between the plunger and the cylinder.
It is another object of the present invention to encapsulate electronic components in a resin molding apparatus wherein the apparatus has a plunger that reduces turbulent flow of the resin and causes the resin to optimally flow towards the desired mold cavities and away from any undesired areas or interfaces within the mold apparatus.
It is a further object of the present invention to provide a plunger that is used in a molding apparatus for encapsulating electronic components wherein the plunger is stably guided within the cylinder.